1. Field of the Invention
This invention relates to integrated circuit fabrication technology and, more specifically, to processes for depositing titanium nitride films via chemical vapor deposition.
2. State of the Art
The compound titanium nitride (TiN) has numerous potential applications because it is extremely hard, chemically inert (although it readily dissolves in hydrofluoric acid), is an excellent conductor, possesses optical characteristics similar to those of gold, and has a melting point around 3000xc2x0 C. This durable material has long been used to gild inexpensive jewelry and other art objects. However, during the last ten to twelve years, important uses have been found for TiN in the field of integrated circuit manufacturing. Not only is TiN unaffected by integrated circuit processing temperatures and most reagents, it also functions as an excellent barrier against diffusion of dopants between semiconductor layers. In addition, TiN also makes excellent ohmic contact with other conductive layers.
In a common application for integrated circuit manufacture, a contact opening is etched through an insulative layer down to a diffusion region to which electrical contact is to be made. Titanium metal is then sputtered over the wafer so that the exposed surface of the diffusion region is coated. The titanium metal is eventually converted to titanium silicide, thus providing an excellent conductive interface at the surface of the diffusion region. A titanium nitride barrier layer is then deposited, coating the walls and floor of the contact opening. Chemical vapor deposition of tungsten or polysilicon follows. In the case of tungsten, the titanium nitride layer provides greatly improved adhesion between the walls of the opening and the tungsten metal. In the case of the polysilicon, the titanium nitride layer acts as a barrier against dopant diffusion from the polysilicon layer into the diffusion region.
At least five processes are presently used for creating thin titanium nitride films: (1) reactive sputtering; (2) annealing of a sputter-deposited titanium layer in a nitrogen ambient; (3) a high-temperature atmospheric pressure chemical vapor deposition (APCVD) process, using titanium tetrachloride, nitrogen and hydrogen as reactants; (4) a low-temperature APCVD process, using ammonia and tetrakis-dialkylamido-titanium compounds which have the generic formula Ti(NR2)4 as precursors; and (5) a low-pressure chemical vapor deposition (LPCVD) process, using tetrakis-dialkylamido-titanium compounds as the sole precursor. FIG. 1 depicts the structural formula of tetrakis-dialkylamido-titanium. Each of the xe2x80x9cRxe2x80x9d groups may be as simple as a methyl group (CH3xe2x80x94) or they may be more complex alkyl groups. Each of the processes enumerated above has its associated problems.
Both reactive sputtering and nitrogen ambient annealing of deposited titanium result in films having poor step coverage, which are not useable in submicron processes. Chemical vapor deposition processes have an important advantage in that conformal layers of any thickness may be deposited. This is especially advantageous in ultra-large-scale-integration circuits, where minimum feature widths may be smaller than 0.3 xcexcm. Layers as thin as 10 xc3x85 may be readily produced using CVD. However, TiN coatings prepared using the high-temperature APCVD process must be prepared at temperatures between 900-1000xc2x0 C. The high temperatures involved in this process are incompatible with conventional integrated circuit manufacturing processes. Hence, depositions using the APCVD process are restricted to refractory substrates such as tungsten carbide. The low-temperature APCVD, on the other hand, though performed within a temperature range of 100-400xc2x0 C. that is compatible with conventional integrated circuit manufacturing processes, is problematic because the precursor compounds (ammonia and Ti(NR2)4) react spontaneously in the gas phase. Consequently, special precursor delivery systems are required to keep the gases separated during delivery to the reaction chamber. In spite of special delivery systems, the highly spontaneous reaction makes full wafer coverage difficult to achieve. Even when achieved, the deposited films tend to lack uniform conformality, are generally characterized by poor step coverage, and tend to deposit on every surface within the reaction chamber, leading to particle problems. A problem with the LPCVD process is that the deposited TiN films are high in carbon content even if the precursor is limited to tetrakis-dialkylamido-titanium, the compound of the group which contains the fewest carbon atoms. The carbon atoms within the precursor molecules are incorporated into the film as the precursor molecules dissociate. Although it is possible to reduce the carbon content of the films by annealing them in ammonia and nitrogen gases, the films attain neither the purity nor the conductivity of sputtered films.
What is needed is a new chemical vapor deposition process which will provide highly conformal TiN films of high purity and with step coverage that is suitable for sub-0.25 xcexcm generations of integrated circuits.
This invention includes various processes for depositing titanium nitride films containing less than 5 percent carbon impurities and less than 10 percent oxygen impurities by weight via chemical vapor deposition and the use of a metal-organic precursor compound. Sheet resistance of the deposited films is within a range of about 1 to 10 ohms per square. The deposition process takes place in a deposition chamber that has been evacuated to less than atmospheric pressure and utilizes the organo-metallic compound tertiary-butyltris-dimethylamido-titanium (TBTDMAT) and a nitrogen source as precursors. The compound tertiary-butyltris-dimethylamido-titanium has the formula (CH3)3CTi(N(CH3)2)3. FIG. 2 depicts the structural formula of tertiary-butyltris-dimethylamido-titanium. It will be noted that one dimethylamido group of the tetrakis-dimethylamido-titanium molecule has been replaced with a tertiary butyl group. The tertiary butyl group is more easily removed from the molecule not only because it is larger than the dimethyl amido group, but because the carbon-titanium bond is weaker than the nitrogen-titanium bonds. The resultant molecule is more reactive than tetrakis-dimethylamido-titanium and, in a chemical vapor deposition reaction, should produce films having a lower percentage of carbon impurities. The deposition temperature, which is dependent on the nitrogen source, is within a range of 350xc2x0 C. to 700xc2x0 C. The low end of the temperature range utilizes nitrogen-containing gases such as diatomic nitrogen, ammonia, amides, amines and hydrazine which have been converted to a plasma. The higher end of the temperature range relies on thermal decomposition of the nitrogen source for the production of reaction-sustaining radicals. In such a case, the use of diatomic nitrogen gas is precluded because of its high dissociation temperature.
Other materials may be incorporated in the titanium nitride films during either embodiment of the deposition process as heretofore described. For example, a titanium nitride film incorporating aluminum and having the general formula TiAlN may be deposited by introducing aluminum-containing compounds such as aluminum chloride (AlCl3) dimethylethylamidoalane (DMEAA) or dimethylaluminumhydride (DMAH) along with the TBTDMAT and nitrogen-containing compounds. Additionally, a titanium nitride film incorporating tungsten and having the general formula TiNW may be deposited by introducing tungsten halide compounds such as WF6 or WCl6 or an organo-metallic compound such as bis(2,4-dimethylpentadienyl)tungsten along with the TBTDMAT and silicon-containing compounds. The aluminum-containing compounds or tungsten-containing compounds may be introduced in a manner similar to that of the other reactants using the temperature guidelines heretofore provided for each embodiment of the invention.